The present invention relates generally to optical encoders. More particularly, the present invention relates to improved optical encoders having higher contrast than prior art encoders.
Optical encoders detect motion and typically provide closed-loop feedback to a motor control system. When operated in conjunction with a code scale, an optical encoder detects motion (linear or rotary motion of the code scale), converting the detected motion into digital signal that encode the movement, position, or velocity of the code scale. Here, the phrase “code scale” includes code wheels and code strips.
Usually, motion of the code scale is detected optically by means of an optical emitter and an optical detector. The optical emitter emits light impinging on and reflecting from the code scale. A typical code scale includes a regular pattern of slots and bars that reflect light in a known pattern. Light is either reflected or not reflected from the code scale. The reflected light is detected by the optical detector. As the code scale moves, an alternating pattern of light and dark corresponding to the pattern of the bars and spaces reaches the optical detector. The optical detector detects these patterns and produces electrical signals corresponding to the detected light, the electrical signals having corresponding patterns. The electrical signal, including the patterns, can be used to provide information about position, velocity and acceleration of the code scale.
FIG. 1A illustrates a cross sectional side view schematic of a known optical encoder 100 and a code scale 120. FIG. 1B is the code scale 120 as viewed from the optical encoder 100. FIGS. 1A and 1B include orientation axes legend for even more clarity.
Referring to FIGS. 1A and 1B, the encoder 100 includes an optical emitter 102 and an optical detector 104 mounted on a substrate 106 such as a lead frame 106. The optical emitter 102 and the optical detector 104 as well portions of the lead frame 106 are encapsulated in an encapsulant 108 including, for example, clear epoxy. The encapsulant 108 defines a first dome-shaped surface 110 (first lens 110) over the optical emitter 102 and a second dome-shaped surface 112 (second lens 112) over the optical detector 104.
The optical emitter 102 emits light 114 that leaves the encapsulant 108 via the first lens 110. The first lens 110 concentrates or directs the emitted light 114 toward the code scale 120, the light reflecting off of the code scale 120. The reflected light 116 reaches the optical detector 104 via the second lens 112. The second lens 112 concentrates or directs the reflected light toward the optical detector 104. The optical detector 104 can be, for example only, photo detector that converts light into electrical signals.
The shape and the size of the first lens 110 and the second lens 112 are dictated by various factors such as, for example only: the distance of the code scale 102 from the lenses 110 and 112 and the characteristics of the emitter 102 and the detector 104.
Often, space 118 between the lenses 110 and 112 is filled with the same encapsulant 108 material and has a flat surface 117. The flat surface 117 presents a surface from which stray light such as stray light 119 from the emitter 102 reflects to impinge on the detector 102 as reflected stray light 121. Such stray light 119 is not desired because stray light that reach the detector 102 introduces false signals, lowers resolutions at which the desired signals can be analyzed.
Accordingly, there remains a need for improved optical encoder that alleviates or overcomes these shortcomings.